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Starting current vs. Running current

Date : April 28, 2023

Starting current and running current respond quite differently to changes in supply voltage. A drop in line voltage results in a lower starting current and a higher running current. A 5% increase in line voltage improves starting current while significantly decreasing operating current. If the line voltage climbs by more than 5%, the starting and running currents will rise as well.

In comparison to the running condition, the induction motor draws a large beginning current. The starting current of an induction motor is approximately six times that of the motor's full load current.

When a three-phase supply is applied to the induction motor's stator, the motor uses magnetising current to create a rotational magnetic flux in the air gap. The flux passes through the air gap, and the magnetic flux is cut off by the short-circuited rotor conductor. When the rotor conductor cuts the magnetic flow, a voltage is induced. The current begins to flow through the rotor conductor. The torque is created by the interaction of the rotor current and the main flux.

Starting and running current are the two most essential parameters in determining the rating of an AC induction motor. In electrical engineering, starting current, abbreviated as Istart, is a measure or rating of how much current is necessary to start a single or three-phase AC motor. It is usually four times the flow rate. Similarly, running current, often known as Irun, is the current required for an AC induction motor to run freely with no load after it has been started. Similarly, full-load current, often known as ifull-load, is the current necessary for an AC induction motor to operate at full load or rated power.